To meet higher capacity demands and to enhance user experience, cellular communications networks are increasing the number of base stations employed. One approach for increasing the density of base stations is achieved by cell splitting macro cells in highly loaded geographical areas. Specifically, the macro cell may be split into multiple small cells in highly loaded geographical areas. These highly loaded areas may be considered traffic hotspots within the coverage area of the macro cell. This densification of the underlying support for the cellular network may allow radio resources to be reused. Additionally, because wireless devices may be closer to the serving base station, wireless devices may achieve higher bitrates.
Another approach for meeting high capacity demands is to employ a mixture of macro cells and small cells with overlapping coverage areas within the cellular network. This type of cellular network may be referred to as heterogeneous networks (HetNets). Such networks may be an important complement to macro cell splitting. One example includes a cellular network having clusters of pico cells within the macro coverage area to offload macro traffic. A pico base station provides service to a pico cell. Typically, a pico base station is a low power node (LPN) that transmits with low output power and covers a much smaller geographical area than a high power node, such as a macro base station. Other examples of low power nodes are home base stations and relays.
Though the presence of additional base stations increases system performance and improves user experiences, such networks are not without its disadvantages. One such disadvantage may be that the wireless devices served by the network may experience lower geometries. As a result, downlink inter-cell interference may be more pronounced and the achievable bit rates may be limited. To mitigate inter-cell interference, mitigation techniques have been employed to improve user performance. Such techniques may explore the structure of the physical layer transmission of the radio access technology.
Interference mitigation may take place on the transmitter side, the receiver side, or on both sides. Interference mitigation on the transmitter side includes those methods that seek to coordinate the physical channel transmissions across cells to avoid severe interference. For example, an aggressor base station may occasionally mute its transmissions on certain radio resources in order for a victim base station to schedule interference sensitive wireless devices on radio resources with reduced interference.
LTE features that seek to coordinate transmissions, on the network side, have been specified in the context of inter-cell interference coordination (ICIC) and coordinated multipoint transmissions (CoMP). In the case of ICIC, for example, a network node such as an eNodeB may send a message over the LTE inter-eNB interface (X2). The message may include coordination information that a receiving network node, such as another eNodeB, may use when scheduling interference sensitive wireless devices. As such, competing transmissions may be coordinated to avoid inter-point interference. As another example, CoMP may use a cluster of transmission points, or base stations, to jointly and synchronously transmit the same signals to a and thereby increase the received power on the desired signals.
The following ICIC messages over X2 have been specified in TS 36.423:                Uplink (UL) Overload Interference Indication (OII) indicates the interference level (low, medium, high) per resource block (RB) experienced by the indicated cell on all RBs.        UL High Interference Indication (HII) indicates the occurrence of high interference sensitivity per RB, as seen from the sending eNodeB.        Received Narrow Transmit Power (RNTP) indicates per RB whether DL transmission power is lower than the value indicated by a threshold.        Almost Blank Subframe (ABS) pattern indicates the subframes on which the sending eNodeB will reduce power for some physical channels and/or reduced activity.        
The X2 messages OII, HII and RNTP represent methods for coordinating physical data channel transmissions in the frequency domain across cells. In contrast, the ABS message is a time domain mechanism to primarily protect reception of PDCCH, PHICH and PDSCH in the small cells by letting macro cells occasionally mute or reduce transmit power on PDCCH/PDSCH in certain subframes. The eNodeB ensures backwards compatibility towards wireless devices by continuing transmission of necessary channels and signals in the ABS pattern for acquiring system information and time synchronization.
On the receiver side, advanced receivers employing enhanced interference suppression schemes, maximum likelihood techniques and interference cancellation techniques are gaining popularity. Such advanced receivers operate to mitigate downlink (DL) interference arising from neighbor-cell transmissions to wireless devices in neighboring cells. Specifically, such receivers may explicitly remove all or parts of the interfering signal.
Generally, such receivers may be categorized into 3 families:                Linear receivers whose aim is to suppress the interference by exploiting an explicit channel estimation of the interfering signal(s).        Non-linear receivers such as ML detection (iterative or non-iterative).        Interference Cancellation (IC) receivers which explicitly cancel the interference from the received signal. IC receivers may be linear or non-linear, iterative or        
One specific type of receiver may use interference rejection combining (IRC) for mitigating inter-cell interference. IRC is a technique for suppressing interference, which requires estimation of an interference/noise covariance matrix. Another type of receiver for mitigating interference may include interference cancellation (IC) receivers that operate to estimate unwanted signals (intra/inter-cell interference). As an example, an IC receiver in the victim wireless device may operate to demodulate and optionally decode the interfering signals, produce an estimate of the transmitted and the corresponding received signal, and remove that estimate from the total received signal to improve the effective signal-to-noise ratio (SINR) for the desired signal. In post-decoding IC receivers, the interfering data signal is demodulated, decoded, its estimated contribution to the received signal is regenerated, and subtracted. In pre-decoding receivers, the regeneration step is performed directly after demodulation, bypassing the channel decoder. The preferred mode to perform such cancellation may include applying soft signal mapping and regeneration rather than hard symbol or bit decisions. Additionally or alternatively, maximum likelihood receivers may be used to jointly detect the desired signals and the interference signals in accordance to the maximum likelihood criterion. Iterative maximum likelihood receivers may be defined to exploit the decoding of the interfering signals.
Both IRC and IC are wireless device reference receiver techniques in LTE. However, IC in LTE is restricted to cancellation of always-on signals, such as the CRS, in which the network assists the wireless device on how these signals are transmitted in the aggressor cells. The two interference cancellation approaches differ by the achievable cancellation efficiency. Stated differently, the fraction of the impairment power left after the cancellation operation may be essentially equal in some scenarios and vary significantly in others. While the post-decoding IC approach may provide superior performance at “high” SIR operating points, these approaches have differing computational resource requirements. For example, the described post-decoding solution implies turbo decoding processing. Additionally, the processing delay incurred may vary by technique. For example, the post-decoding solution requires buffering the entire code block of the interfering signal.
To apply these advanced interference cancellation techniques to signals originating from other cells, knowledge of certain signal format parameters may be required to configure the receiver. For pre-decoding IC, the resource allocation, modulation format, any pre-coding applied, the number of layers, etc. may be useful, and may be obtained via blind estimation, eavesdropping other-cell control signaling, or via NW assistance features. For post-decoding, additional transport format parameters are required which may typically only be obtained from receiving or eavesdropping the related control signaling.
However, the different types of receivers may require differing information and/or parameters and are required to estimate blindly all the parameters needed for the receiver implementation. Additionally, the multitude of communication standards applicable to LTE may include many features which may need to be supported by the wireless device but which will not be used by a network (depending on the configuration) and may make blind detection difficult and complex. Currently no signaling is defined in LTE standard in order to provide wireless devices with the assistance which may be needed in order to implement advanced receivers with limited complexity.